Anti-lock control of a four-wheel drive vehicle is coventionally based upon the vehicle speed. In a two-wheel drive vehicle, the vehicle speed substantially corresponds with the speed of a driven wheel (the speed of a rear wheel in a front-wheel drive vehicle) and hence, during acceleration, the vehicle speed can be determined based upon this driven-wheel speed. In contrast, in a four-wheel drive vehicle, it is difficult to determine the vehicle speed when the wheel(s) spin due to increasing speed of the wheels on a snowy road, a frozen road or the like, because the four-wheel drive vehicle has no driven wheels.
If the vehicle speed during acceleration of the wheels cannot be determined with precision, as described above, the accuracy of the anti-lock control may also be reduced.